At its present stage of development, the precision machining field is in a state of flux. Systems that are totally dependent on manual operations have largely given way to techniques whereby manufactured parts are made with general-purpose, numerically controlled machining systems. Although cutting or other removal of material occurs automatically in such systems, numerous manual operations are still required, primarily for measuring the machined dimensions and for making cutter adjustments using an ordinary numerical control cutter offset. These manual measurements and adjustments of the cutting tool are necessary to take into account a large number of variables, such as wear of the cutting tool, repositioning and/or replacement of the cutting tool, as well as dimensional changes of the cutting tool, of the work piece and of the machining apparatus itself due to such factors as heating, deflection under load and the like.
By way of example, in a typical operation carried out with a numerically controlled machine tool such as a lathe, certain adjustments, i.e. tool offsets, must be manually implemented by the operator after the machine is set up for the manufacture of a particular work piece or part. Prior to the start of machining the operator must advance the cutting tool to a tool setting surface and determine the tool position by manually measuring the space between the tool and the reference surface. This is normally done with a piece of shim material or the like, and such measurements then form the basis for manually making tool offsets. Where the lathe includes tool holding means such as a multiple tool turret, this operation must be carried out separately for each tool as well as for each of the axes (of motion) of the machine. Prior to making the final or finishing cut for a particular work piece surface, the various dimensions of the semi-finished work piece surface are measured by using a hand-held gauge. This enables the operator to determine the required offset of the cutting tool which is used for the finishing cut. After the finishing cut is made, the work piece is again checked with the hand-held gauge in order to measure the conformance of the actual dimensions of the finished surface to the desired dimensions.
The manual operations described above are individually time-consuming and take up a significant amount of the total time required to machine a particular work piece to the desired dimensions. This serves to limit the manufacturing capacity of the machine tool. Considering present day costs of a lathe or a milling machine (machining center), any reduction of the capacity of the machine tool becomes a matter of economic significance. Further, all such manual operations further open the manufacturing process to human error.
As is generally recognized, the solution to the foregoing problems is to automate manual measurements and the manual adjustments of the cutting tool, e.g. by the use of a computer-operated numerical control system. In such a system the computer may either be positioned remote from the numerical control unit, or it may be incorporated in the latter, e.g. in the form of a microcomputer. Alternatively, a computing capability may be provided remote from the numerical control unit as well as being incorporated into the latter. Instead of down-loading successive blocks of data stored on tape or the like, as is the case in an ordinary NC system, a computer numerical control (CNC) system is capable of storing entire programs and calling them up in a desired sequence, editing the programs, e.g. by addition or deletion of blocks, and carrying out the computations of offsets and the like.
Although fully automatic systems have not been widely adopted at this stage of development of the precision machining field, a considerable amount of development work has been done to date, much of it limited to special purpose situations wherein a single machining operation is repetitively carried out. It is also known to mount a tool sensor in the form of a touch trigger probe on the bed of the machining apparatus, or on a pivotal arm that can be swung out of the way when desired. The position of the cutting tool can be calibrated against such a probe by noting the tool position when contact with the probe occurs. From the observed deviations between the programmed and the actual positions, a compensating offset may be determined and stored in the memory associated with the computer numerical control means. The offset compensates for the difference between the programmed contact position and the actual contact position.
A system and method which incorporates the features described above is disclosed in a copending application by William A. Hunter and Allan R. Barlow, entitled "System and Method of Precision Machining," Ser. No. 283,850, filed July 16, 1981, U.S. Pat. No. 4,382,215 which is assigned to the assignee of the present application. As disclosed in the copending application, a touch trigger probe or part sensor is mounted in the tool holding means. The latter probe is first calibrated against datum or reference surfaces and is subsequently used to calibrate the tool sensor probe. Only then is the cutting edge of the selected tool calibrated by contact with the tool sensor probe. The initial tool offsets which are determined from the results of this operation are stored in numerical control means. After machining has taken place, the part sensor probe is again calibrated and is then used to probe the machined surface(s) of the work piece. The information so obtained determines the final offsets required for the finishing cut. Subsequently, the finished surface may be probed to determine its conformance with the desired dimensions.
The system and method described above use a touch trigger probe for both tool sensing as well as part sensing. Although simple in construction, the touch trigger probe must be configured for a class of specific features to be probed. The probes themselves, which are normally purchased as commercial products from specific vendors, tend to be fragile and they occupy at least one tool position on the tool holding means, e.g. on the lathe turret.
Other approaches which have been proposed for the form of closed loop machining under discussion here involve the use of lasers, eddy current, ultrasonics and conductivity probes. While all of these approaches can be made to work, they all have severe practical limitations in an actual shop operating environment.
Laser systems require line-of-sight mechanisms to position the probe. It is difficult to build such a device which is universally applicable and at the same time cost effective. Laser systems also have problems with surface reflectivity, particularly when cutting fluids are used.
Conductivity probes have insulating requirements for certain elements of the machine tool, such as the turret. Another problem associated with such probes is that of passing electrical signals through elements which constitute conductors of less than optimum conductivity, such as spindle bearings and slip rings.
Eddy current and ultrasonic probes have stringent requirements with respect to the proximity of the probe and the work piece, as well as stringent coupling requirements.
All such probes must further be articulated and configured for specific work piece configuration, e.g. where an internally machined surface must be measured. Thus, in general the systems described are complex, difficult to implement, and difficult to render cost effective by making them universally applicable to different types of precision machining situations.